Generally, sliding surface lubricating oil compositions used for machine tools and lubricating oil compositions for metal working such as cutting oil and grinding oil differ of their compositions from each other. However, ordinary machine tools are in such designs so that both of these oil compositions are apt to mix with each other, resulting in the problems of reduction of performance and unsatisfactory function due to the mixing of different oils.
The applicants have proposed a lubricating oil composition for combined use which comprises a lubricating oil such as mineral oil, synthetic oil or the like and a specific long chain carboxylic acid to solve the above problems (Japanese Patent Unexamined Publication (Kokai) No. 141793/86). This lubricating oil composition is an excellent oil composition for combined use which is substantially free from the above problems, but further improvement in sliding surface characteristics is required.